This invention relates in general to aperture couplers, and more particularly, asymmetric stripline aperture couplers.
In applications where a microstrip patch antenna is used, coupling to the patch antenna typically presents an undesirable manufacturing process. One method that is commonly used makes direct contact to the patch antenna with a center conductor of a coaxial cable, with a shield of the coaxial cable connected to a ground plane associated with the patch antenna. This method is not compatible with an automated surface mount assembly process. A plated via can be used in place of the center conductor of the coaxial cable, but a layer of dielectric material must be used between the patch antenna and the ground plane associated with the patch antenna. If a low cost, typically lossy, dielectric is used, a reduction in antenna efficiency results.
To avoid manufacturing problems and the loss of antenna efficiency associated with direct contact methods, non-contacting methods have been developed. These methods rely on electromagnetic field coupling techniques which are compatible with surface mount assembly processes and are essentially lossless. One non-contacting method which has been used is known as a proximity feed. Microstrip line, extended beneath the patch antenna for a short distance, provides a coupling mechanism through the parasitic capacitance existing between the microstrip line and the patch antenna. Radiation from the microstrip line has the undesirable effect of degrading the radiation pattern of the patch antenna.
One solution to eliminating the undesired degradation of the radiation pattern of the patch antenna due to the proximity feed is an aperture fed patch antenna in which the patch antenna and the microstrip line are separated by a microstrip ground plane with a small opening. The opening serves as an aperture aligned with the microstrip conductor beneath the patch. A non-transverse electromagnetic (non-TEM) field formed in the vicinity of the aperture couples the patch antenna to the microstrip line. The aperture fed patch antenna may be used with a simple radio transceiver which does not demand multilayer circuit board techniques to support higher system complexity.
To support higher system complexities, multi-layer circuit board constructs require stripline rather than microstrip. A simple extension of the aperture fed patch antenna method using stripline transmission line simply provides an additional ground plane separated from the microstrip line, now a stripline conductor, by a dielectric. The addition of another ground plane has the benefit of isolating the stripline conductor from additional layers, but also provides an image of the non-TEM field formed by the aperture in the opposite ground plane. The image degrades the intensity of the non-TEM field and thus reduces the coupling effects of the non-TEM field.
One method for suppressing the degrading effects of the image replaces the dielectric in the vicinity between the stripline conductor and the aperture with a dielectric plug of substantially higher dielectric constant than the dielectric between the stripline conductor and the ground plane with the aperture. The higher dielectric constant of the plug concentrates electric field intensity between the stripline conductor and the ground plane in the vicinity of the aperture. This enhances the non-TEM field used for coupling, and suppresses the image field which degrades coupling.
Placing the high dielectric plug between the stripline conductor and the ground plane in the vicinity of the aperture complicates the assembly and increases the number of parts required to produce the assembly, both of which add to the cost of the assembly. An easily manufactured low cost stripline aperture coupler with enhanced coupling would be beneficial for use in multilayer circuit board applications.